In order to measure and control temperature accurately, thermometer sensors are used that have a high degree of sensitivity and linearity in the required temperature range. The ultimate resolution of the measuring instrument is determined by the measuring chain and the physical characteristics of the sensor.
A widde variety of physical phenomena have been used to provide temperature probes capable of operating over various temperature ranges. For example, probes based on the principle of expansion in bodies that are solid, liquid (alcohol, mercury) or gaseous (hydrogen) have been used, as have probes based on visible radiation (pyrometers). Other instruments rely on temperature variations in electrical characteristics in resitors, thermistors, and thermocouples.
At present, the highest performance temperature measuring instruments rely on methods that cause the frequency of an oscillator to vary with temperature. In particular, it has been known for some years that plates of quartz used as frequency standards in high stability oscillators are subject to temperature effects that perturb their accuracy.
Since quartz is anisotropic, crystal cut orientations can be found to minimize, or alternatively to selectively increase sensitivity to temperature.
A quartz orientation that can be used to produce plates with a linear temperature coefficient over a wide temperature range is described by D. L. Hammond, C. A. Adams & P. Schmidt in a paper entitled "A linear quartz crystal temperature sensing element" given at the 19th annual conference of the ISA in October 1964. This bulk wave quartz cut, known as the LC (Linear Coefficient) cut, has been used to make a quartz thermometer by inserting the plate in an amplified loop to obtain an oscillator whose frequency varies linearly with temperature (D. L. Hammond &. A. Benjaminson in "Unthermometre lineaire a quartz" (A linear quartz thermometer), in the February 1966 issue of the journal "Mesure"). Measurements could be performed automatically with a resolution of 10.sup.-4 .degree. C. over a temperature range of -40.degree. C. to +230.degree. C. A major drawback of such a probe is that its response time is about 10 seconds, which is due to the fact that the bulk wave quartz plate is held by three fixing points which are the seat of the main heat exchangers. The sensitivity of this probe is about 35.times.10.sup.-6 /.degree. C., which corresponds to a frequency variation of 10.sup.3 Hz/.degree. C. at an operating frequency of 28 MHz (3rd partial).
Preferred embodiments of the present invention reduce the response time of such a temperature probe and increase the accuracy of temperature measurement.